Vehicles equipped with an antilock braking system (antilock control function or ABS) and a traction control system (TCS) employ a pressure control unit. Such a device has been disclosed in the Publication of Unexamined Japanese Patent Application Number 57-22948.
That device uses an ABS hydraulic circuit as the basic circuit, wherein a control valve, which operates as a function of the operation of the master cylinder, is installed in the return pathway of the basic circuit, and a pump piston for the traction control system is added to the ABS hydraulic pump. When the master cylinder is activated, this control valve enables passage between a switching valve, switchable between supply or exhaust positions, and the hydraulic fluid reservoir, and when the master cylinder is not operating, then the control valve blocks this passage between the said switching valve and the reservoir, and at the same time links the said supply or exhaust switching valve and the delivery line of the hydraulic pump. As well, hydraulic fluid for both ABS and TCS operation is derived in common from the hydraulic pump; when the TCS is in operation, hydraulic pressure is supplied from the hydraulic pump through the return line of the ABS hydraulic circuit to the appropriate wheel cylinder via the supply-exhaust switching valve.
The pressure control unit as disclosed in that patent has the following problematic points.
(1) The hydraulic pump requires pump pistons for both the ABS and the TCS functions, moreover a 2-position-3-way selector valve is required which means that the device is very complex and therefore expensive.
(2) During operation of the TCS, both the supply of hydraulic pressure from the hydraulic pump to the wheel cylinder and the exhausting of the pressure from the wheel cylinder to the reservoir must proceed through at least two valves including the control valve and the supply-exhaust valve; this mechanism is complex and the overall response of the system is poor.
(3) When pressure is being reduced during operation of the TCS, the hydraulic fluid in the wheel cylinder circulates back to the master cylinder reservoir via the master cylinder channel and the master cylinder. This passage of the hydraulic fluid inside the master cylinder can easily damage the cylinder cup.